Studies on N, N'-dimethylnitrosourea-induced brain tumors in rats--especially, on microtumors.

The incidences, distribution and histopathological findings of N, N'-dimethylnitrosourea (DMNU)-induced brain microtumors in Sprague-Dawley rats were studied. Subcutaneous injections of DMNU in young adult rats one a week resulted in the induction of 122 gliomas in 38 animals with an incidence of 69% after a time lapse of between 157 and 246 days from the first injection. Of these tumors, 66 were classified as microtumors (diameter less than about 1 mm) by detailed light microscopy observation of serial sections. The microtumors were of 3 types: 55 oligodendrogliomas, 8 astrocytomas and 3 mixed gliomas. As the tumors became larger in size, anaplasia appeared, especially in the central part of the tumors. The microtumors developed randomly throughout the brain. It was concluded that, in adult rat brains, the target cells of DMNU were well differentiated glial cells which had already migrated from the matrix layer.